Friday, 9 January 2015

Retrochallenge 2015/01: 4K Color Computer Programming Contest Entries

I have submitted a variety of programs to Allen Huffman's 4K Color Computer Programming Contest. These programs were originally designed to run on the Radio Shack MC-10 with 16K ram pack. I had to "condense" many of them quite a bit to get them to work on the 4K CoCo since it provides even less RAM (2343 bytes) to BASIC than even the MC-10 without the 16K ram pack (3142 bytes). Some of the programs use the speedup poke 65495,0. This will always be located in line 0, so make sure to change this poke to poke 65497,0 if you are going to try to run these programs on a Coco 3.
Farfall is a text-mode arcade game based on John Linville’s Fahrfall. This game was already 4K, so I just tightened up the code a little bit and switched the key sensing peek routine so that it worked on a Coco 1. Whether a key is being pressed can be sensed directly on the MC-10 by looking at the result of PEEK(17023)ANDPEEK(2). If a key is being pressed the result will be the ASCII value of the key, otherwise the result will be zero. On the Coco you must look at a number of different memory locations that constitute a "keyboard rollover table." For example, PEEK(345) will return 255 if no key is being pressed and another value if the Space Bar is pressed. The table for all the keys is held at peek locations 338-345 (these represent different collections of keys on the keyboard). You can either search this table for the specific values returned by the key you are checking for (which don't correspond to ASCII values), or you can search the whole table for any change from the default 255 value and then check PEEK(135) to find the ASCII value of the last key pressed.  However, it should be noted that in the Coco 2B Tandy modified the ROM so that the values of the table were not reset automatically after keys were released. To save space (and speed), and since the Coco 1 doesn't do this, I do not reset these values. So if you run these programs on a Coco 2, moves will continue even after you stop pressing keys. The Coco 3 returned to the original method.
JimVaders is a text-mode Space Invaders style game, with a stationary mother ship that makes a random appearance. The game speeds up as you go. This was already 4K ready. I had designed it to fit into limited memory when someone on the Yahoo MC-10 group mentioned that they only had an MC-10 without the 16k ram pack. The ram pack is essentially the "standard" way to run an MC-10, either in its real or emulated form. However, MC-10s often turn up in junk shops or on the Net without this essential item. People pick these computers up because they look cute and retro and then discover that they also have to track down the much more difficult to locate ram pack if they are to be able to run any descent software. They then become frustrated and their interest wanes. I thought it might help keep some of these folks involved in retro-computing with the MC-10 if I made some games that worked for its most basic layout.
Lander is a version of the classic Lunar Lander done with the 64×32 block graphics. Unlike my original version I had to remove the random and changing Earth generation routine (which was a quixotic feature that I quite liked) and all the instructions. Prompting had to be minimized and I had to scour the net for a line drawing routine that only uses integer math, since Micro Color BASIC lacks mach functions like SIN and COS. I found the Bresenham algorithm, which nicely fit the bill.
Tetris is a version of the classic falling block game. It was one of my earliest programs for the MC-10. However, I abandoned working on it after John Dionne made an excellent machine language hi-res graphic version for the 20K MC-10, which included human voice and sampled sound effects!  As a result, I always had a sense that it could have been improved so I appreciated the opportunity to return to it to see if I could whittle it down to 4K. However, with all its DATA statements detailing the orientations of all the blocks which had to be loaded into a fairly large array it was too large to fit in a 4K Coco. So I used the technique I developed to eliminate the redundancy of loading DATA statements into large array variables for text adventure info described in another of my blog postings. The result is that the program must be loaded in two chunks. You must first load and run the program file T1.BAS and then load the data file T2. If working from a real cassette T2 obviously should be saved after T1. Just leave the tape play button on as you run T1. To create the T2 data file on a real Coco or emulator use the TDAT.BAS file. Use Space and B keys to rotate blocks clockwise or counter-clockwise.
G2048 is a 4K version of the current hit 2048 game (web-based and iPhone/Android). The original game is open source and so I used the posted JAVA code as reference to make it play like the original. I also had to find an alternative to the LOG function, which I had used in the original to calculate the changing colors, since there is no LOG function in Color BASIC.
R1.BASR1.BAS Loading Demo

Rail like Tetris is loaded in 2 parts. Load and run R1.BAS first. Then load data file R2. After that a prompt will appear. This is for the number of trains you would like to control. There is no error checking so make sure to enter only a number between 1 and 6. Use the RDAT.BAS program to create the R2 data file on a real Coco or emulator. The object of the game is to maneuver the blue train in the upper right to the yellow siding in the bottom left. Press the letters to curve or straighten the track at that point. If trains collide or run out of track the game is over. The prompt will reappear for how many trains you would like to control. This game is a port of a game originally for the Commodore Pet. Here’s a link to my blog posting detailing my production of the original for the MC-10.


Asteroid Storm is a game that someone on the Yahoo MC-10 group described as a cross between Frogger and Asteroids. You must maneuver your ship from left to right across a moving asteroid field. However, your reverse thrust engines are broken. But don't despair, pressing the Space key will deploy limited shields to help you escape difficult situations, that is, if you haven't used them up already! This program uses a circle drawing routine for low res block graphics to create asteroids of various sizes. Then it basically snips the top line and prints it at the bottom of the screen to create the illusion of a moving field by taking advantage of auto screen scrolling. As levels are completed new asteroids are added to the field creating more challenge. Along the way you can collect replacement ships (up arrows) and extra points (@).

These programs can be found in my Color Computer program compilation ZIP file. They are contained in the JG4K.dsk file in the ZIP.

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